I have both and at lower volume levels there is a slight difference but mostly only noticeable in an A/B comparison. For me I hear a slight change on female vocals with the 9 sounding like the vocalist has their hands "cupped" around her mouth that isn't there with the 4, or the 2bsst that I have as well. This is with my Gallo Refs and isn't as noticeable with any of my other speakers.
At louder levels the 4 sounds more dynamic with better control of the lowest registers. Having recently purchased a pair of Middle T's, the dynamics are more apparent as well as the increased bass control with the 4. Again, this is at louder volume levels with more dynamic material. The 9 is very satisfying but the 4 is more of everything. At lower levels I actually really prefer the 2b, there is just something about that amp that brings out detail like no other. If this is a 2 channel setup for you, I would go with the 4b and be done with it. If you already have the 9, I would just relax and enjoy it.
